Here is a likely USA census listing taken just before WWII. If you research John's brother & sister, you may find more living relatives.
"United States Census, 1940," database with images, FamilySearch(https://familysearch.org/ark:/61903/1:1:KW24-YK9 : accessed 22 November 2016), John D Landers in household of Obidiah R Landers, Joshua, Justice Precinct 3, Johnson, Texas, United States; citing enumeration district (ED) 126-16, sheet 5A, line 32, family 120, Sixteenth Census of the United States, 1940, NARA digital publication T627. Records of the Bureau of the Census, 1790 - 2007, RG 29. Washington, D.C.: National Archives and Records Administration, 2012, roll 4082.
Good luck! -NGP